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Pneumatic motion control — using compressed air to power actuators, air motors, grippers, and the like — is a long-proven, time-tested technology. Pneumatics applications are found in most any industry, and performance requirements and design parameters can vary widely.

In life sciences, for example, compressed air is used in patient care devices like respirators. They require high precision and controllability, and often light weight and small size. At first glance, a rail-car braking system may appear far removed from a respirator, but it also requires reliability and controllability despite extreme temperatures and dirty environments.

These examples illustrate both the versatility and variety of pneumatic systems. And designers naturally focus on actions and outputs when developing a compressed-air circuit, considering factors such as function, size, energy consumption, power, and capacity.

Regardless of application requirements, proper air preparation is a must if the system is going to perform as required for the longest possible service life. Here’s a look at what designers should consider to select the right filters, regulators, and lubricators to keep systems running efficiently and hassle free.

Filtration First, keep in mind that air produced by a compressor is typically hot, wet, and dirty. The first step in good air preparation is to filter out contaminants that interfere with proper operation and shorten equipment life.

Moisture vapor in the air exiting a compressor outlet will condense to liquid as the air cools. This can reduce efficiency and capacity. Water is most efficiently removed at the lowest possible air temperature and highest pressure. Thus, air should be cooled with an aftercooler near the compressor outlet, along with dripleg drains, automatic drain valves, or filters to remove water upstream of any pressure-reducing valves. A properly sized general-purpose filter will remove liquid water.

Removing liquid water does not remove water vapor from the air. Applications ranging from paint spraying to locomotive braking may require that all vapor be removed. To do this, air dryers are needed. The three principle types are refrigerant, regenerative adsorbent, and deliquescent absorbent dryers, and they vary in terms of drying capabilities and operating costs. (For more information on air dryers from our Fluid Power Basics section, click here.) Also note that all dryers will be compromised if contaminated by liquid water, oil, or water/ oil emulsions, so they should always be used in tandem with filters and air coolers.

Solid particle contaminants can be introduced through the ambient air or by corrosion or wear in the compressor. Most standard air-line filters remove coarse particles 40 μm and larger. Fine particle filtration (10 to 25 μm) is required for high-speed pneumatic tools and process-control instrumentation. Filtration ≤10 μm is essential for air bearings and miniature pneumatic motors. And for tasks such as paint spraying, breathing air, and food-safe applications, particle removal below 1 μm is essential. High-efficiency coalescing filters are required to remove such fine particles from the air stream.

Generally, it is inadvisable to provide finer filtration than is absolutely necessary because fine filter elements trap more dirt and become blocked more rapidly. But when needed, use standard air-line filters as prefilters to avoid overburdening high-efficiency elements with coarse particles.

Oil, another contaminant in compressed air, can exist in three forms — an oil/water emulsion, aerosol, or oil vapor. Standard air-line filters can remove emulsions.

Aerosols — small particles between 0.01 and 1 μm in size — can only be removed by special coalescing filters. These are typically rated by the amount of air they can process at a given cleanliness level, normally a maximum remaining oil content of 0.01 ppm in the exit air. Flow that exceeds the rating will not only increase pressure drop across the unit (and, therefore, energy costs) but, more importantly, remaining oil content will increase. Again, protect coalescing filters from particulate and water contamination with air-line filters mounted immediately upstream.

Oil vapor is typically present in such minute quantities that it can be ignored except in sensitive applications like food and beverage processing, pharmaceuticals, and breathing air. Passing air through an adsorbing bed of activated carbon, after flowing through standard and coalescing filters, removes oil vapor.

Once all contaminants have been considered, the degree of cleanliness for each machine or part of a plant can be determined. Using the proper filter in the right location minimizes energy and maintenance costs. Always determine the volume of air involved in each stage as undersized, inappropriate filters are a prime cause of high energy costs.